U.S. patent application number 12/381238 was filed with the patent office on 2009-09-03 for high brightness light emitting diode with a bidrectionally angled substrate.
This patent application is currently assigned to NeosemiTech Corporation. Invention is credited to Myung-Hwan Oh, Soo-Hyung Seo, Joon-Suk Song.
Application Number | 20090218562 12/381238 |
Document ID | / |
Family ID | 36931270 |
Filed Date | 2009-09-03 |
United States Patent
Application |
20090218562 |
Kind Code |
A1 |
Song; Joon-Suk ; et
al. |
September 3, 2009 |
High brightness light emitting diode with a bidrectionally angled
substrate
Abstract
A light emitting diode includes a substrate tilted toward first
and second directions simultaneously, a first cladding layer formed
with a semiconductor material of a first conductive type on the
substrate, an active layer formed on the first cladding layer, and
a second cladding layer formed with a semiconductor material of a
second conductive type on the active layer, wherein
concavo-convexes are formed on the interfaces of the first cladding
layer, the second cladding layer, and the active layer, and the
(100) substrate is a III-V or a IV-IV group semiconductor
substrate, and has a crystal orientation such that a (100) plane of
the (100) substrate is inclined 2 to 20.degree. toward the [0-1-1]
direction and 1 to 8.degree. toward the [0-11] direction.
Inventors: |
Song; Joon-Suk;
(Gyeonggi-do, KR) ; Seo; Soo-Hyung; (Gyeonggi-do,
KR) ; Oh; Myung-Hwan; (Gyeonggi-do, KR) |
Correspondence
Address: |
CHAPMAN AND CUTLER
111 WEST MONROE STREET
CHICAGO
IL
60603
US
|
Assignee: |
NeosemiTech Corporation
Seoul
KR
|
Family ID: |
36931270 |
Appl. No.: |
12/381238 |
Filed: |
March 10, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11359798 |
Feb 22, 2006 |
7524708 |
|
|
12381238 |
|
|
|
|
Current U.S.
Class: |
257/13 ;
257/E33.008 |
Current CPC
Class: |
Y10S 438/973 20130101;
H01L 33/16 20130101; H01L 33/22 20130101 |
Class at
Publication: |
257/13 ;
257/E33.008 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 28, 2005 |
KR |
10-2005-0016770 |
Dec 1, 2005 |
KR |
10-2005-0116268 |
Claims
1. A light emitting diode comprising: a (100) substrate having a
crystal orientation such that a plane of the substrate is inclined
toward a first and a second direction simultaneously; a first
cladding layer formed with a semiconductor material of a first
conductive type on the (100) substrate; an active layer formed on
the first cladding layer; and a second cladding layer formed with a
semiconductor material of a second conductive type on the active
layer, wherein concavo-convexes are formed on the interfaces of the
first cladding layer, the second cladding layer, and the active
layer; and wherein the (100) substrate is a Ill-V or a IV-IV group
semiconductor substrate; and wherein the (100) substrate has a
crystal orientation such that a (100) plane of the (100) substrate
is inclined 2 to 20.degree. toward the [0-1-1] direction and 1 to
8.degree. toward the [0-11] direction.
2. The light emitting diode of claim 1, wherein the (100) substrate
is a GaAs substrate, and the first and second cladding layers are
formed with InGaAlP single crystal thin films, and the active layer
includes InGaP quantum wells and InGaAIP quantum barriers.
3. The light emitting diode of claim 1, wherein the (100) substrate
is an InP substrate, and the first and second cladding layers and
the active layer are formed with InGaAsP single crystal thin
films.
4. The light emitting diode of claim 1, wherein the (100) substrate
is a SiC substrate.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a division of U.S. patent application Ser. No.
11/359,798, filed on Feb. 22, 2006. This application claims
priority to and the benefit of Korean Patent Applications Nos.
10-2005-0016770 and 10-2005-0116268 filed respectively on Feb. 28
and Dec. 02, 2005 in the Korean Intellectual Property Office, the
contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] (a) Field of the Invention
[0003] The present invention is related to a light emitting diode
and a fabrication method thereof, and in particular, to a light
emitting diode fabricated by growing single crystal thin films
through controlling the crystal orientation of the substrate.
[0004] (b) Description of the Related Art
[0005] A light emitting diode is a photoelectric conversion element
having a structure such that an n-type semiconductor which has
electrons as a carrier and a p-type semiconductor which has holes
as a carrier are welded to each other.
[0006] In order to convert electrical energy into light, a voltage
is set up on both electrode terminals of the light emitting diode,
and a current is applied in one direction. The electrons and the
holes recombine and a part of the energy generated by the
recombination is converted into light.
[0007] The light emitting diode is used for a display when the
wavelength of the light emitted from the light emitting diode is in
the visible region, it is used for various excitation light sources
when the wavelength is in the ultraviolet region, and it is used as
an element that transfers information between machines when the
wavelength is in the infrared region.
[0008] The performance of a light emitting diode that emits visible
light is determined by light emitting efficiency (luminous
efficiency), and the light emitting efficiency is indicated as
lumens per watt.
[0009] The light emitting efficiency of the light emitting diode is
mainly dependent on the three factors of internal quantum
efficiency, extraction efficiency, and operating voltage.
[0010] The internal quantum efficiency is a characteristic value of
the amount of photons for the amount of electrons passing through
the light emitting diode, and it is determined by the quality of
the semiconductor material and the design of the active
portion.
[0011] The extraction efficiency is a ratio of the amount of
photons extracted from the semiconductor chip to the amount of
photons generated in this way.
[0012] Many deflections of the photons occur due to a large
difference of the refractive index between the semiconductor and
the other material, and some of the photons are absorbed by the
semiconductor chip during the deflection. Accordingly, the
extraction efficiency is limited by the deflection of the photons
in the semiconductor chip.
[0013] The external quantum efficiency is a product of the internal
quantum efficiency and the extraction efficiency.
[0014] Recently, InGaAIP and InGaN light emitting diodes that were
grown with metal organic chemical vapor deposition have been
developed. InGaAIP is a Ill-V group compound semiconductor of
direct transition that can be matched to a GaAs substrate and that
has high internal quantum efficiency.
[0015] In spite of the development of technologies, the efficiency
of extraction of photons that can be extracted from the inside to
the outside of the light emitting diode, that is, the extraction
efficiency, has a fundamental limitation due to the internal loss
and the large refractive index of the light emitting diode.
[0016] Methods for improving light extraction efficiency are under
constant development. As one of these efforts, a method of forming
concavo-convexes on the substrate is known.
[0017] The concavo-convexes are formed on the substrate through a
photolithography and dry etching technique, and single crystal thin
films are grown on the substrate which the concavo-convexes are
formed on.
[0018] A diffused reflection of light emitted from the active layer
occurs due to the concavo-convexes formed on the interfaces of the
single crystal thin films so that the light emitting efficiency is
improved.
[0019] However, the process of forming the concavo-convexes on the
substrate is very complex because of the photolithography and dry
etching technique. In addition, it is difficult to grow the single
crystal thin films on a plane that has the concavo-convexes, and
the cost of fabricating the light emitting diode is too high.
SUMMARY OF THE INVENTION
[0020] It is an object of present invention to provide a light
emitting diode that has high emitting efficiency, and in particular
that has high extraction efficiency, and a fabrication method of
the same.
[0021] This object may be achieved by a light emitting diode and a
fabrication method of the same with the following features.
[0022] The light emitting diode includes a substrate having a
crystal orientation such that a plane of the substrate is tilted
toward a first and a second direction simultaneously, a first
cladding layer is formed with a semiconductor material of a first
conductive type on the substrate, an active layer is formed on the
first cladding layer, and a second cladding layer is formed with a
semiconductor material of a second conductive type on the active
layer, wherein concavo-convexes are formed on the interfaces of the
first cladding layer, the second cladding layer, and the active
layer.
[0023] In this case, the substrate may be a III-V or a IV-IV group
semiconductor material.
[0024] The substrate may be a (100) III-V group semiconductor
substrate having a crystal orientation such that a (100) plane of
the substrate is tilted toward [0-1-1] and [0-11] directions
simultaneously.
[0025] The substrate may have a crystal orientation such that the
(100) plane may be tilted as 2 to 20.degree. toward the [0-1-1]
direction and 1 to 8.degree. toward the [0-11] direction.
[0026] The substrate may be a GaAs substrate, the first and second
cladding layers may be formed with InGaAIP single crystal thin
films, and the active layer includes an InGaP quantum well and an
InGaAIP quantum barrier.
[0027] The substrate may be an InP substrate, and the first and
second cladding layers and the active layer may be formed with
InGaAsP single crystal thin films.
[0028] The substrate may be a SiC substrate.
[0029] The fabrication method of a light emitting diode includes
steps of preparing a substrate that is tilted toward a first
direction and a second direction simultaneously, forming a first
cladding layer with a semiconductor material of a first conducive
type on the substrate, forming an active layer on the first
cladding layer, and forming a second cladding layer with a
semiconductor material of a second conducive type on the active
layer.
[0030] The steps of forming the first cladding layer, the second
cladding layer, and the active layer may be performed with
metal-organic chemical vapor deposition or molecular beam epitaxy,
or with the mixed method of both the metal-organic chemical vapor
deposition and the molecular beam epitaxy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a sectional schematic view of the light emitting
diode according to the exemplary embodiment of the present
invention.
[0032] FIG. 2 is a schematic view showing the crystal orientation
of the fabrication method of the light emitting diode according to
the exemplary embodiment of the present invention.
[0033] FIGS. 3A and 3B are schematic views showing the different
surface step of the (100) GaAs substrate.
[0034] FIG. 4 is a schematic view showing the diffused reflection
of the light by single crystal thin films.
[0035] FIG. 5 is a graph showing the change of surface roughness
according to the tilted angle in an experimental example.
[0036] FIG. 6 is atomic force microscope (AFM) images showing the
surface roughness of the grown single crystal thin films as the
(100) GaAs substrate is tilted toward 2nd-direction.
[0037] FIG. 7 is a graph showing the change of the integrated
optical intensity according to the surface roughness of the single
crystal thin films in the experimental example.
[0038] FIG. 8 is a graph showing a spectrum of the
photoluminescence (PL) of the single crystal thin films grown on
the tilted (100) GaAs substrate.
[0039] FIG. 9 is a graph showing the shift of PL near band edge
(NBE) energy, the change of a full width half maximum (FWHM), and
the change of the PL intensity of the single crystal thin films in
the experimental example.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] The present invention will be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown.
[0041] FIG. 1 is a sectional view showing the structure of a light
emitting diode according to an exemplary embodiment of the present
invention.
[0042] As shown in the drawing, the light emitting diode includes a
substrate 10 having a predetermined crystallographic orientation, a
first buffer layer 12 formed on the substrate 10, a first cladding
layer 14 formed on the first buffer layer 12, a first confining
layer 16 formed on the first cladding layer 14, an active layer 18
formed on the first confining layer 16, a second confining layer 20
formed on the active layer 18, a second buffer layer 22 formed on
the second confining layer 20, a second cladding layer 24 formed on
the second buffer layer 22, a window layer 26 formed on the second
cladding layer 24, and a cap layer 28 formed on the window layer
26.
[0043] The first buffer layer 12, the first cladding layer 14, and
the first confining layer 16 are formed with a semiconductor
material of a first conductive type, and the second cladding layer
24, the window layer 26, and the cap layer 28 are formed with a
semiconductor material of a second conducive type.
[0044] In addition, a first electrode 30 of the first conducive
type is arranged beneath the substrate 10, and a second electrode
32 of the second conducive type is arranged on the cap layer
28.
[0045] The light emitting diode according to the exemplary
embodiment is an InGaAlP light emitting diode.
[0046] In this case, the substrate 10 may be a (100) GaAs
substrate, the first buffer layer 12, the first cladding layer 14
and the first confining layer 16 may be formed with n-type InGaAIP,
and the active layer 18 may include quantum wells (QW) formed with
GaInP and quantum barriers (QB) formed with InGaAlP.
[0047] The second confining layer 20 and the second buffer layer 22
may be formed with n-type InGaAIP, the second cladding layer 24 may
be formed with p-type InGaAIP, the window layer 26 may be formed
with p-type InGaAIP or p-type GaP, and the cap layer 28 may be
formed with p-type GaAs.
[0048] Meanwhile, the first electrode 30 may be formed with n-type
Au/AuGeNi or Au/AuGe, and the second electrode 32 may be formed
with p-type Au/AuBe or Au/AuZn.
[0049] The light emitting diode according to another exemplary
embodiment of the present invention may be an InP light emitting
diode. In this case, the substrate may be an InP substrate, and the
first cladding layer, the second cladding layer, and the active
layer may be formed with InGaAsP.
[0050] However, the present invention may be adapted to other light
emitting diodes in addition to the InGaAIP and InP light emitting
diodes described above. In these cases, semiconductor substrates
such as III-V and IV-IV group substrates including a SiC substrate
can be used.
[0051] FIG. 2 is a schematic view showing a crystal orientation of
the substrate 10. The substrate 10 has a crystal orientation such
that a plane of the substrate is tilted toward a first direction
and a second direction simultaneously.
[0052] As shown in the drawing, the substrate 10 is a (100) GaAs
substrate, and the substrate 10 has a crystal orientation such that
the (100) plane of the substrate is tilted as 2 to 20.degree.
toward the [0-1-1] direction and 1 to 8.degree. toward the [0-11]
direction.
[0053] If single crystal thin films are grown on the substrate 10
having such crystallographic orientation, concavo-convexes are
formed on the interfaces between the single crystal thin films and
the substrates. Consequently, the extraction efficiency of the
light emitting diode is improved by the diffused reflection of the
light emitted from the active layer 18.
[0054] A fabrication method of the light emitting diode will be
described hereinafter.
[0055] As described above, the substrate 10 of the light emitting
diode according to the exemplary embodiment has a crystal
orientation such that the (100) plane of the substrate is tilted as
2 to 20.degree. toward the [0-1-1] direction and 1 to 8.degree.
toward the [0-11] direction.
[0056] Tilting the (100) GaAs substrate 10 toward the [0-1-1]
direction is for preventing degradation of the single crystal thin
films grown on the (100) GaAs substrate 10 due to the ordering.
[0057] However, in the case that the substrate 10 is tilted as less
than 2.degree. toward the [0-1-1] direction, the effect of
preventing the ordering is small. Accordingly the (100) plane of
the substrate 10 is tilted as 2.degree. or more toward the [0-1-1]
direction.
[0058] In addition, in the case that the substrate 10 is tilted as
more than 20.degree. toward the [0-1-1] direction, the crystal
orientation of the substrate 10 becomes different from that of the
(100) plane. Accordingly the (100) plane of the substrate 10 is
tilted as 20.degree. or less.
[0059] In this way, if the single crystal thin films are grown on
the substrate 10 in the state that the (100) plane of the substrate
10 is tilted as 2 to 20.degree. toward the [0-1-1] direction and
then is tilted toward the [0-11] direction again, Ga and As atoms
are exposed simultaneously on the surface of the substrate 10.
Accordingly, the interfaces of the single crystal thin films become
rough.
[0060] Therefore, the concavo-convexes do not exist in the
substrate 10 itself. However, the concavo-convexes are generated on
the interfaces of the single crystal thin films formed on the
substrate 10, so the light emitting efficiency is enhanced.
[0061] The process with which the concavo-convexes are generated on
the interfaces of the single crystal thin films will be described
more fully hereinafter.
[0062] FIG. 3a shows a surface step of the (100) GaAs substrate in
the case that the (100) GaAs substrate is cut while being tilted
toward the [0-1-1] direction, and FIG. 3b shows a surface step of
the (100) GaAs substrate in the case that the (100) GaAs substrate
is cut while being tilted toward the [0-11] direction.
[0063] As shown in FIG. 3a, when the substrate is cut while being
tilted toward the [0-1-1] direction, the Ga atoms are exposed on
the step. The tilted plane of the substrate has similar properties
to that of a (111)A plane.
[0064] Meanwhile, as shown in FIG. 3b, when the substrate is cut
while being tilted toward the [0-11] direction, the As atoms are
exposed on the step. The tilted plane of the substrate has
properties similar to that of a (111)B plane.
[0065] Therefore, in this way, according to the tilting direction
of the substrate, the optimal growth condition of the single
crystal thin films is changed.
[0066] In the case that the substrate is tilted toward the [0-1-1]
direction and the [0-11] direction simultaneously as in the
exemplary embodiment, the Ga and As atoms are exposed on the
surface of the substrate at the same time. It therefore deviates
from the optimum crystal growth condition, and accordingly the
interfaces of the single crystal thin films become rough as they
grow.
[0067] In this way, the concavo-convexes are generated on the
interfaces of the single crystal thin film during the growth of the
single crystal thin film without an additional process when
selectively controlling the crystal orientation of the
substrate.
[0068] FIG. 4 schematically shows the diffused reflection of the
light in the single crystal thin films grown through the process
described above.
[0069] When the substrate 10 is tilted as less than 1.degree.
toward the [0-11] direction, the effect of generating the
concavo-convexes on the interfaces of the single crystal thin films
is small. Accordingly the (100) plane of the substrate 10 is tilted
as 1.degree. or more toward the [0-11] direction.
[0070] However, when the substrate 10 is tilted as more than
8.degree. toward the [0-11] direction, the ordering effect becomes
serious. Accordingly, the (100) plane of the substrate 10 is tilted
as 1 to 8.degree..
[0071] The present invention will be described more fully
hereinafter with experimental examples. However, the experimental
examples are merely for exemplifying the present invention, and the
present invention is not restricted thereto. Experimental
Examples
[0072] InGaP thin films with a 4000 .ANG. thickness were grown on
each of 4 inch (100) GaAs substrates tilted as 15.degree. toward
the [0-1-1] direction, and 0.degree., 4.degree., and 6.degree.
toward the [0-11] direction, respectively. The change of the
surface roughness (morphology) of the InGaP single crystal thin
film and the integrated optical intensity according thereto were
analyzed.
[0073] In addition, the 10K photoluminescence (PL) characteristic
of the InGaP single crystal thin films were analyzed. Near band
edge (NBE) energy transfer, the change of PL intensity, and the
change of integrated optical intensity were measured.
[0074] In the experimental example, the growth temperature was
650.degree. C., the V/III ratio was 150, the growth rate was 2
mm/hr, and the InGaP single crystal thin film was evaporated by
metal-organic chemical vapor deposition (MOCVD).
[0075] FIG. 5 is a graph showing the change of surface roughness
according to the tilted angle in an experimental example, and FIG.
6 is atomic force microscope (AFM) images showing the surface
roughness of the grown single crystal thin films as the (100) GaAs
substrate is tilted toward 2nd-direction.
[0076] As shown in FIG. 5, as the tilting angle was increased, the
surface of the single crystal thin film became rougher. This means
that the concavo-convexes were generated on the interfaces during
the growth of the single crystal thin film. This can be confirmed
through the AFM images of FIG. 6.
[0077] FIG. 7 is a graph showing the change of the integrated
optical intensity according to the surface roughness of the single
crystal thin films in the experimental example, FIG. 8 is a graph
showing a spectrum of the photoluminescence (PL) of the single
crystal thin films grown on the tilted (100) GaAs substrate, and
FIG. 9 is a graph showing the shift of PL near band edge (NBE)
energy, the change of a full width half maximum (FWHM), and the
change of the PL intensity of the single crystal thin films in the
experimental example.
[0078] As shown in FIG. 7, as the surface of the single crystal
thin film becomes rougher. It means that more concavo-convexes
exist on the surface, the integrated optical intensity increases
linearly.
[0079] In addition, as shown in FIG. 8, as the tilting angle of the
substrate toward the [0-11] direction increased from 0.degree. to
4.degree. and 6.degree., the main peak was red-shifted. And as
shown in FIG. 9, NBE of PL transited to low energy.
[0080] This means that ordering occurred in the InGaP single
crystal thin film. That is, as the direction of the substrate was
changed from 0.degree. to 4.degree. and 6.degree., the ordering
occurs remarkably. In addition, the enlargement of the full width
half maximum (FWHM) of PL shows that the crystallinity of the
single crystal thin film was degraded.
[0081] Generally, it is known that the ordering imparts a negative
effect on the characteristics of InGaP optical elements, but FIG. 8
and FIG. 9 show that not only the extent of ordering but also the
quantity of the light was increased.
[0082] This means that although the crystallinity was degraded due
to the ordering, the extraction efficiency was improved according
to the diffused reflection of the light generated from the InGaP
single crystal thin film.
[0083] Although preferred embodiments of the present invention have
been described in detail hereinabove, it should be clearly
understood that many variations and/or modifications of the basic
inventive concept herein taught which may appear to those skilled
in the art will still fall within the spirit and scope of the
present invention, as defined in the appended claims.
[0084] As described above, the present invention provides a light
emitting diode having improved light emitting efficiency by
selectively controlling the crystal orientation of the
substrate.
[0085] In addition, the present invention provides a fabrication
method of the light emitting diode with a relatively low production
cost by such a simple process.
* * * * *